78 SCENERY OF THE HEAVENS. 



The crescent moon presents a singular appearance in the sky. Under. favourable 

 circumstances, the whole lunar circlet may then be seen, the dark part appearing of 

 somewhat smaller dimensions in proportion to the illuminated. The appearance is popu- 

 larly described as that of the new moon with the old one in her arms. It arises from 

 the light reflected from the earth to the lunar surface, hence called earthshine, and 

 lumiere cendree by the French, or ashy light, on account of its inferiority in quantity 

 and brightness, to that which is directly received from the sun. It only serves to render 

 the unenlightened portion of the moon very faintly visible ; and the dark part of her 

 body appears disproportionate to the size of the crescent, owing to the optical illusion 

 which the presence of a strong light creates, that of apparently augmenting the magni- 

 tude of objects. Two causes contribute to render the dark portion of the lunar disk 

 invisible in other stages of her progress : the increase of her directly illuminated part 

 diffusing a stronger light, which proportion ably nullifies that which is reflected from the 

 earth ; and the actual diminution of the earthshine itself. When the moon is a crescent to 

 us, the earth is about full to her, and, consequently, more light is then transmitted from 

 the earth than in other circumstances, which has the effect of then bringing that portion 

 of her disk, not exposed to the solar rays, into feeble visibility. The effect is not 

 produced when the moon is half full, owing to the cause, for the reasons stated, being 

 less influential. 



The moon is situated in external space at a mean distance of 237 thousand miles 

 from the earth. Great as this interval is when compared with terrestrial extent, it is 

 only about ^-^th part the earth's distance from the sun, and little more than one fourth the 

 diameter of the solar body. It is owing to this proximity to us, that she occupies so large 

 a space in the heavens, for the lunar diameter is only 2160 miles. Our own globe is 

 equal in magnitude to forty-nine such bodies, and the sun to near seventy millions. If 

 loosened from the action of other forces, the earth and the moon would fall together by 

 the power of mutual attraction ; but the earth being not only the larger body but the 

 most dense, and its attraction being far the most powerful, the moon would descend to it, 

 passing the intervening space in less than, five days, our own planet courteously 

 advancing about the distance of its semidiamefer to meet the satellite. To the lovers of 

 singular coincidences, the following may be acceptable, respecting the three bodies with 

 which we are principally concerned. 



Miles. 



Diameter of the Moon, 2160x110 = 237,600, average mean distance from the earth. 

 Diameter of the Sun 870,320 x 1 10 = 95,735,200, average mean distance from the earth. 

 Diameter of the Earth, 7912 x 1 10 = 870,320, estimated diameter of the sun. 



There are two proper motions belonging to the moon, besides the annual pilgrimage 

 around the great central orb accomplished in company with the earth. If her place with 

 reference to a neighbouring star on any clear night be ascertained, the next night she 

 will be found to have moved about 13 further eastward, and each night the distance will 

 have widened, until from an opposite quarter of the heavens she will appear advancing 

 towards the same proximity to the star. This proves her motion in an orbit round the 

 earth ; but great irregularities belong to it, which accurately to estimate is one of the 

 most difficult problems and highest achievements of astronomy. The lunar tour through 

 the heavens is accomplished in 27 d 7 h 43 m 1 1 s , after which time, she returns to nearly 

 the same position in relation to the stars. But the interval between one conjunction with 

 the sun and another, or her synodical period, the lunar month, is 29 d 12 h 44 m 2 s , because 

 of her partnership in the orbital motion of the earth. To compare and reconcile the 

 motions of the sun and moon with each other, bringing a certain number of months and 

 years to coincide, was an object to which the ancients applied themselves with great care 



